Focus error detection device for an optical recording/playing back system

Abstract

A focus error detection device to be used in an optical system for recording and/or playing back information, including a light detector made up of four light receiving elements arranged symmetrically with respect to the center of the light detector, a multiplication means for generating a pair of multiplication signal by multiplying output signals of two light receiving elements which are arranged symmetrically with respect to the center of the light detector. The multiplication signals are applied to a subtraction means where the focus error signal is generated by the subtraction between the multiplication signals. Thereby the off-set of the focus error signal which has been caused by the deviation of the position of the center of the light beam from the center of the light detector is by far reduced.

What is claimed is: 
     
       1. A focus error detection device to be used in an optical system for recording/playing back information on and from a record medium, comprising: a light detector having first to fourth light receiving elements which are arranged on the side of two boundary lines crossing substantially at right angles with each other;   an optical means for directing a reflection light beam from a surface of the record medium to a light receiving surface of said light detector in such a manner as to provide an astigmatic focus; and   an operation circuit means for deriving a focus error signal, including first multiplication means for generating a first multiplication signal by multiplying with each other output signals of said first and third light receiving elements which are arranged symmetrically with respect to a center of said light detector, second multiplication means for generating a second multiplication signal by multiplying with each other output signals of said second and fourth light receiving elements which are arranged symmetrically with respect to the center of said light detector, and a subtraction means connected to said first and second multiplication means for generating a differential signal between said first and second multiplication signals.   
     
     
       2. A focus error detection device as set forth in claim 1, further comprising a summation means connected to said light receiving elements for generating a sum signal by adding output signals of said first to fourth light receiving elements, and a dividing means connected to said subtraction means for dividing said differential signal with said sum signal.